The present invention concerns circuits for the deflection of the electron beam in a television picture tube and, more specifically, a circuit arrangement for the protection of the final stage of a monolithic integrated circuit power amplifier designed to power the vertical deflection control of the picture tube.
A vertical deflection circuit for a television picture tube usually includes an oscillator, a sawtooth signal generator, a yoke driver amplifier and a flyback signal generator. Such circuits can be built on a single silicon chip by applying the common manufacturing techniques used for monolithic integrated circuits. A vertical deflection circuit of this type is described in detail, for example, in the article entitled: "A MONOLITHIC INTEGRATED CIRCUIT FOR VERTICAL DEFLECTION IN TELEVISION RECEIVERS", submitted by A. Romano and L. Venutti to the 1973 Chicago Fall Conference and published in February 1975 in Transactions on Consumer Electronics, p. 85-93.
A limitation in the use of integrated circuits for vertical deflection power amplifiers is the relatively low power that can be supplied by the final stage and, particularly, by the relatively low power that the output transistor can handle, the output transistor having the load located between collector and emitter thereof. The output transistor is actually the circuit component subjected to the greatest stress in terms of voltage and power. Specifically, during the normal operation of the amplifier, its collector-emitter voltage periodically reaches twice the circuit power supply voltage. In order to prevent current from flowing through the transistor under such voltage conditions, which would mean subjecting the transistor to particularly severe stress, or having to resort to higher power transistors which require greater surface areas on the silicon chip, a usual prior art technique is to cause the transistor, under overvoltage conditions, to operate with the lowest possible impedance between its base and emitter, that is, under conditions approaching those involving the highest breakdown voltage, called by the experts BV.sub.CES (shorted-base collector-to-emitter breakdown voltage). For example, in the circuit described in the publication referred to above, a protective transistor is inserted between the base and the emitter of the final power output transistor which, through an appropriate connection to the amplifier output, is placed in saturation as soon as the output voltage level exceeds the circuit power supply voltage. In this manner, the final power output transistor, having a very low base-emitter impedance, is inactive and therefore is able to withstand the maximum power dissipation and voltage permitted by its physical and geometric features.
In some cases, however, even with the usual prior art protection techniques, the final power output transistor, before the intervention of the protective transistor, passes a current which, when combined with a relatively high collector-emitter voltage, causes it to operate at power levels which are very close to the safety limits. In any case, the final power output transistor is subjected to stressful conditions for long periods of time, which may cause a rapid deterioration of the transistor, thereby resulting in the failure of the entire integrated circuit. In order to reduce this effect, it is necessary for the protective transistor to become saturated before the amplifier output voltage reaches the power supply voltage level. A known system to obtain this result consists of obtaining a pulse from the oscillator which controls the sawtooth signal generator, the pulse having the same period as the sawtooth signal and starting at the end of the front, or ramp portion, of said signal. The obtained pulse is used to cause the saturation of the protective transistor. In this manner, since the amplifier output voltage reaches the power supply voltage level only after the end of the ramp, the final power output transistor is in a state having a minimum base-emitter impedance and having a collector-emitter voltage which is lower than the power supply voltage.
Obtaining a pulse from the oscillator to operate the protective transistor in the manner described above may be convenient when the oscillator is integrated in the same silicon chip that contains the power amplifier, but is no longer so when it is necessary to resort to a special terminal for connection to an outside oscillator. In any case, the protection thus obtained occurs at predetermined cyclic intervals, and therefore has no effect in the case of output overvoltages due to accidental interruptions in the ramp.